Starting in 2009, the United States has been the largest natural gas producing country in the world as a result of advanced drilling technologies enabling economical access to the gas from shale deposits. The increased collection of natural gas from shale deposits offers several benefits including economic growth and improved energy independence. However, the collection also creates potential environmental issues, one of which is emissions of volatile organic compounds (VOCs) from the natural gas condensate tanks. Also, VOCs may react with NOx in the presence of sunlight to form smog, which may cause serious human health issues and affect visibility. To address the potential environmental impact, the Environmental Protection Agency (EPA) has issued regulations that require the oil and gas storage and condensate tanks to reduce the VOC emissions by 95% if the VOCs emissions are greater than 6 tons/yr.
In addition to the VOCs released from oil and gas storage and condensate tanks, other waste VOCs, such as, for example, biogas from landfills or waste gas from chemical plants, need to be addressed, and the exhaust after combustion must satisfy current environmental regulations. Landfill waste generates gas consisting of about 40% to about 60% methane, with the rest of the gas primarily being carbon dioxide. Conventionally, the gas is collected and sent to a flare or oxidizer to prevent the release of methane into the atmosphere. These waste gas streams may not have sufficient heating value to sustain the combustion, and a significant amount of supplemental fuel may be needed to achieve a clean incineration process. The need for supplemental fuel not only increases the operational cost to reduce methane emissions but also results in increased greenhouse emissions.
A 95% reduction in VOC emissions is conventionally typically achieved by use of a vapor recovery unit (VRU) or by use of a combustion control device in a method known as flaring.
A VRU is able to recover the waste VOCs and transport them in a pipeline for sale or for use on-site, but the capital and operational costs are high.
In cases where VOC recovery is difficult or not economically feasible, combustion control devices are the most common solution. An open flare is the most economical and common thermal oxidation system used in the field. However, since the new EPA regulations require stricter emission standards and no visible emissions for the storage or condensate tank combustion device, an enclosed flaring system is considered to be a more feasible option to meet the new stringent requirements. In addition, the new regulations require a flame to be present at all times. Supplemental fuel is therefore needed for incinerating low heating value VOCs and for sustaining the pilot flame, which increases the operational cost. To reduce this fuel consumption and therefore the cost, heat recovery may be considered. In one type of conventional VOC incinerator, called a regenerative thermal oxidizer (RTO), two ceramic beds alternately recover heat and preheat the incoming flow. A 95% heat recovery can be achieved and a destruction efficiency greater than 98% has been reported. While the high heat recovery of RTOs provides significant energy benefits, a drawback is the need to constantly switch the flow direction between the two ceramic beds with valves. These moving parts typically increase maintenance costs. In addition, RTOs have a high capital cost to manufacture, making them not very cost effective for condensate tank emission control applications that have relatively low vent flow rates.
The biogas generated by landfill waste is conventionally incinerated in an enclosed flare. While this method successfully prevents the release of methane, there are two major challenges associated with enclosed flares. First, the waste gas and air are non-premixed. This results in hot reaction spots during combustion that increases the exhaust concentration of NOx, which must be maintained less than 0.06 lb/MMBTU. Second, when the methane makes up less than 40% of the waste gas, the reaction instability may cause incomplete destruction, in which case supplemental fuel is needed for complete destruction.